The proportion of benzene currently in both leaded and unleaded petrol is about 1% by volume in the United Kingdom.  There has been no increase in the amount of   benzene in leaded petrol over the last decade, in spite of a threefold reduction in its lead content.

          While people working with petrol, in its manufacture and distribution, might be expected to derive their main exposure from this source, a major source of exposure for the general population is from vehicle exhausts, and here benzene is mostly produced by chemical reactions occurring during combustion of petrol in the engine.  The importance of this source is illustrated in Table 2, which shows that no less than 78% of atmospheric benzene in the United Kingdom is emitted from petrol engine exhausts.  From this it is clear that to achieve air quality standards for benzene , control from this source is essential. While petrol exhaust is the major source of benzene in the air we breathe outdoors, especially in urban areas, it is important to realize that there are other sources of human exposure, some of which contribute a significant proportion of an individual’s total intake.  Cigarette smoke contains benzene, and may be the main source of exposure for a heavy smoker; passive smoking may make a small contribution to benzene intake.  Benzene may also be present in low concentrations in some foods and in drinking water, and diet may be the main source of benzene for non-smokers living in unpolluted, rural areas.

Table 2 gives some estimated figures for the different amounts of benzene (measured by weight in microgram i.e. µg ) to which we may be exposed daily from these sources.

Table 2 :  Sources of Benzene emissions in the UK in 1991.

Figures rounded to the nearest 50 tonnes

Source : Department of Environment, U.K.. Expert Panel on Air Quality Standards : Benzene (1994)

Table 3 : World Production of benzene in thousands of tones for 1981

Note : Benzene in Petrol is not included.

Source : WHO, Environmental Health Criteria Document-150, Benzene (IPCS, 1993)

·        Gasoline (Petrol) and Mobile sources :

Benzene is present in both exhaust and evaporative emissions.  Motor vehicles account for approximately 85% of the total benzene emissions.  The remaining is attributed to stationary sources (15%).  Benzene is a component of gasoline. 

Another  important source of benzene in air is active smoking of tobacco.   As per EPA’s estimate active smoking accounts for roughly half of the total population exposure to benzene, which is over and above that from motor vehicles.

Benzene is quite stable in atmosphere and the only important reaction in the lower atmosphere in the reaction with OH radical.  This reaction too is very slow. Benzene is one of the ingredients of petroleum product.

Benzene hydrocarbons can be condensed and washed out to the coke-oven gas. Crude oil contains benzene and its homologues account for 3.9%-4.8%. In USA Benzene is also produced from olefines. Benzene in the motor fuel is infact burnt in the combustion process, but at the same time there is an additional amount of benzene produced through dealkylation which is emitted. Benzene is mainly used as raw material for the production of substituted aromatic hydrocarbons. The major source is emission from motor vehicles and evaporation losses during the

handling distribution and storage of petrol.

As the CPCB survey during 1988-89 five Metro cities account for 35% of total vehicular population in India.  2 Wheelers in 5 Metros only account for 23% of all 2 Wheelers  in India. Two-wheelers have grown in number accounting for 68.8% of all vehicles in 1995 against 8.8% in 1951 in India.

Delhi alone accounts 1/8^th of Total Vehicle Population (TVP) in India.  The total number of vehicles in Delhi (27.0 lacs) was more than number of vehicles in cities of Mumbai (7.24 lacs), Calcutta (5.61 lacs) and Chennai (8.12 lacs) put together during 1995. The total no. of 2 Wheelers in Delhi (14.03 lacs) was about one and half times of other three Metros i.e. Mumbai (2.46), Calcutta (2.22) & Chennai (4.61) put together during 1993.

The total number of vehicles in Delhi was 30 lacs as on March 31, 1998., which has grown up to more than  34.25 lacs as on  July 2001. (Out of which CNG vehicle constitute 2450 Buses, 1178 Mini-Buses, 27,263 Three-Wheeler, 1993 Taxis). Diesel vehicles constitutes about 6% of total vehicles. Two wheelers owing to predominate category (i.e. about 2/3^rd of total vehicular population in Delhi need utmost attention. In the petrol driven two-wheeler the emission of benzene is significant because about 20-30% of fuel is coming out as unburnt hydrocarbon.

Table 4 : Percentage Contribution of Pollution Load From Gasoline And 

                Diesel Vehicles (Comparative Account).

There are two sources of hydrocarbon emissions from the vehicles – evaporative emission and engine exhaust emission.  The evaporative emission from vehicles comprises of running losses, diurnal losses, hot soak losses and fuel loading losses.  The losses arise from the fuel tank and the carburetor.  Similarly in gasoline handling systems from the refinery storage tank to gasoline dispensing stations evaporative losses take place.

            Gasoline consumption in Delhi has shown a growth of 247% during 1997-98 against 1980-81 as compare to Diesel consumption growth of 150% only.

Table 5 : Estimate of Demand of Transport Fuels in India  (‘000 Tonnes)

·        Benzene Emission from Passenger Car :

 Benzene is a major constituent of evaporative emission due to its high volatility.  In the gasoline engine exhaust against benzene is a significant component of hydrocarbon fraction due to its refractory nature in the combustion process.

 Benzene emissions in European cars using modified ECE + EUDC test cycle are correlated by the following equations (ref 52).

For noncatalyst cars

BEE     =            1.515 + 0.765.BF + 0.0414. NBAF (eq1)

For catalyst cars

BEE     =            1.237 + 0.599.BF + 0.0602. NBAF (eq2)

Where BEE – wt% benzene in total hydrocarbons in engine exhaust,

BF       -            wt% benzene in feed and

NBAF  -            wt% nonbenzene  aromatics in feed.

It may be noted that BEE is very sensitive to benzene content in gasoline and also depends on aromatic content in the fuel.

Sources of Benzene Emission :

­